This invention relates to an antenna system and, more particularly, to an antenna system for overcoming the deleterious effect of multipath.
The multipath effect is the result of radio waves reflecting off of surfaces before reaching their destination. The reflections, which occur commonly both indoors and outdoors, vary in strength depending on such factors as their proximity to the transmitter and the surface type of the material off which they are reflecting. The reflections may reach the destination at different times from the main signal and each other, resulting in signal fluctuations. Relatively weak reflections may be insignificant, but stronger reflections may result in undesirable signal quality.
One approach to overcoming the multipath effect focuses on antenna diversity. There are two main design streams for developing diversity arrays. These design streams address the two main cases of transmission in an indoor environment, which are (1) transmitting with a clear line of sight (LOS) between transmitter and receiver and (2) transmitting with an obscured line of sight (OBS).
In the first case, the received signal quality can be optimized when an antenna with a very narrow beam is aimed at the transmitter site. This method may be highly efficient for LOS cases since the LOS signal is generally the strongest of all multipath components, and the narrow beam attenuates all the multipath signals except those in the line of sight.
The disadvantages of the LOS method are related to implementation issues. In order to produce very narrow beams, large antenna arrays are needed. However, large arrays may be difficult to integrate in an indoor wireless product. Moreover, implementing a design that would have four very narrow beams and the ability of covering 180 degrees in the azimuth would dramatically increase the cost of the design. Therefore, an angle diversity scheme is implemented for an indoor wireless product and the use of wide beams cannot be avoided. Since the most severe multipath components have a small angular spacing from the main LOS signal, the limitations of implementing angle diversity in small arrays are quite clear.
In the second case, where the transmission occurs with an obscured line of sight, angle diversity with very narrow beams may be misused. In these cases, the use of wide beam widths and space diversity is more effective. The main idea behind space diversity is to use a number of omni-directional antennas placed a distance apart so that the received signals from each antenna show low correlation. It is expected that the hyperthesis of the different instances of the multipath signals at each antenna element will produce a high signal quality on at least one of the elements. The larger the number of elements, the larger the probability of receiving a signal of high quality.
However, space diversity presents some significant disadvantages. Since omni-directional antennas are used, the elements"" gain is rather low, which means that the distance between transmitter and receiver cannot be extended. Additionally, space diversity cannot decrease the delay spread of the signals received. This means that although the bit rate of a channel using space diversity may be increased, the symbol rate is limited.
Therefore, it is desirable to merge the positive characteristics of the LOS and OBS diversity schemes. It is also desirable to be efficient in terms of cost and size constraints in the construction of an antenna structure.